One transmission systems for the wide-band color picture signals the bandwidth of which is compressed by the multiple sub-sampling, is called the MUSE (Multiple Sub-Nyquist Sample Encoding) system, which is submitted to experimental broad-casting in Japan and is a superior narrow-band transmission system for band-compressed wide-band color picture signals.
The principle of band-compressed narrow-band transmission of this MUSE system consists in that the sub-sampling is effected at sampling points, positions of which are different between mutually adjacent lines, fields or frames, so as to transmit as much picture information as possible by employing as few sample values as possible and further so as to previously remove aliasing distortions caused in a reproduced picture by the sub-sampling efficiently through a prefilter provided in an encoder, which suitably restricts the bandwidth of the picture signal.
In the sub-sampling of the MUSE system, any of inter-line, inter-field and inter-frame sub-sampling are effected for both motional and stationary picture signals according to the principle of employment of fixed density sampling system at a predetermined sampling rate. However, in practice, a somewhat complicated sampling system is carried out according to the combination of several different sampling rates.
As a result, a sampling pattern of the transmission signal from the encoder is a quincuncial sampling pattern in which the arrangement of sampling points in each field is circulated at every four fields so as to reduce the number of samples in each field to be as small as possible. On the other hand, in the decoder receiving the transmitted sample values of quincuncial pattern, with regard to the stationary picture region, the inter-field and the inter-frame inter-polations are effected by utilizing all of the sample values of every four fields in which the sampling pattern is completed, so as to reproduce a substantially completed original picture, while, with regard to the motional picture region, the inter-field interpolation cannot be effected because of the inter-field time difference of 1/60 second which is caused in the picture contents in response to the picture motion, so that the in-field, namely inter-line, interpolation is effected only on the sample values in every one field. The detail of this MUSE system is described in the specifications of Japanese Patent Application No. 58-194,115 (corresponding to U.S. Pat. No. 4,745,459) and Japanese Patent Application No. 60-106,132 (corresponding to U.S. Pat. No. 4,692,801, which are assigned to the present assignee, and the document of NHK Laboratory Note No. 348"Concept of the MUSE System and its Protocol", which is published by the present assignee. In this connection, the outline of the encoder of the MUSE system, which relates to the subject of the present invention, will be described by referring to FIG. 1 hereinafter.
In the encoder of the MUSE system which is outlined in FIG. 1, each primary color picture signals R, G, B of a high definition color television signal, namely, a so-called high-vision, are supplied, through low-pass filters (LPF) 1.sub.R, 1.sub.G, 1.sub.B (each having its pass-band limited below 21 MHz or 22 MHz) to analog-to-digital converters (A/D) 2.sub.R, 2.sub.G, 2.sub.B so as to be digitized at clock rate 48.6 MHz. The outputs from the A/D converters are supplied through a gamma-correction circuit 3 in common to a matrix circuit 4 so as to convert the primary color picture signals R, G, B by matrixing into a luminance signal Y and two kinds of color difference signals C.sub.1 C.sub.2, the former Y being directly supplied to a TCI encoder 6, and the latters C.sub.1, C.sub.2 being supplied to the same encoder 6 respectively through low-pass filters (LPF) 5.sub.cl, 5.sub.c2, pass-bands of which are commonly limited below 8.1 MHz.
TCI encoder 6 is provided for converting the luminance signal Y and the color difference signals C.sub.1, C.sub.2 to a single time-sequencial signal through time-axis compression and time-divisional multiplication, so as to apply time-divisional multiplication upon these three signals by inserting the color difference signals C.sub.1, C.sub.2, which are applied with time-axis compression and then combined alternately in line-sequence, into line fly-back intervals of the luminance signal Y.
The time-sequential picture signal derived from the TCI encoder 6 is separated into a stationary picture sequence S and a motional picture sequence M. The time-sequential picture signal of the stationary picture sequence S is supplied to an inter-field prefilter 7 so as to previously remove signal components in a frequency range in which aliasing distortion is caused by the subsequent sub-sampling and thereafter is supplied to an inter-field offset subsampler 8 so as to apply the inter-field offset subsampling thereon at the sample rate 24.3 MHz, an output sub-sample series picture signal being supplied to a sample rate converter 10 through a low-pass filter (LPF) 9, a pass band of which is limited below 12 MHz, so as to convert the sample rate from 48.6 MHz to 32.4 MHz and thereafter being supplied to a mixer 13. On the other hand, the time-sequential picture signal of the motional picture sequence M is supplied to an in-field pre-filter 11 so as to previously remove signal components in a frequency range in which the aliasing distortion is caused by the subsequent sub-sampling and thereafter is supplied to a sample rate converter 12 so as to convert the sample rate from 48.6 MHz to 32.4 MHz and then is supplied to the same mixer 13.
In the mixer 13, the time-sequential picture signals of the stationary and the motional picture sequences S and M care appropriately mixed in response to the result of motion detection in a motion detector 14 specially provided in the encoder, the mixed output time-sequential picture signal being supplied to an inter-line and inter-frame offset sub-sampler 15, so as to form a sample series picture transmission signal of the MUSE system by applying the inter-frame and interline offset sampling in response to the quincuncial pattern at the sample rate 16.2 MHz.
The encoder of the MUSE system which is recognized as a preferred example of the sub-sample transmission system for the wide-band color picture signal is basically arranged as described above by referring to FIG. 1 and the feature of this arrangement exists in the respect that the sub-sample transmission mode of the picture signal is separated into a stationary picture transmission mode and a motional picture transmission mode in response to the existence of picture motion and the amount thereof, which modes are mixed with each other at an appropriate ratio or are appropriately changed to each other in response to the result of picture motion detection, so as to substantially faithfully reproduce the original picture with the most suitable picture quality.
In the stationary picture transmission mode, the original picture is substantially perfectly reproduced by applying the inter-field and inter-frame interpolation on sampling pattern which is circulated at every four fields, while, in the motional picture transmission mode, the original picture is reproduced with the original picture quality substantially maintained by the in-field interpolation to avoid inter-field time difference of picture contents.
Accordingly, in the encoder having the outlined arrangement as shown in FIG. 1, the pass-band characteristics of the pre-filters 7 and 11, for limiting the transmission bands of the stationary picture signal and the motional picture signal respectively, is set up in conformity with the respective transmission mode. Particularly, with regard to the motional picture transmission mode, the pass-band limitation basically assuming the simple in-field signal processing applied only on picture sample values at every field is effected, so that the picture reproduced by the in-field interpolation corresponding thereto has a difficult problem of picture quality deterioration to be improved, including dimness resulting from the insufficient amount of picture information, even though the required definition is visually mitigated by the picture motion.
An object of the present invention is to solve the above difficult problem and to provide a sub-sample transmission system for a broad band color picture signal in which the transmitted picture quality deterioration caused in the sub-sample transmission system according to the conventional basic arrangement, particularly, that which is caused in the motional picture region mainly assuming the in-field signal processing can be remarkably improved by the broad investigation with regard to various respects.